1. Field of the Invention
The present invention relates to a monitoring device which has a state detection sensor for detecting the state of a monitored object and a motion sensor for detecting the physical motion of a monitored object, a transceiver system which transmits a detection result of the state detection sensor, and a control method of the transceiver system.
2. Description of Related Art
Recently, safety regulations are enhanced in both Japan and the U.S. Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act enforced in North America establishes a new standard that requires the installation of a tire pressure monitoring system (TPMS). The standard applies to new vehicles marketed after 2006. There are currently two types of TPMS: direct and indirect. Direct measurement systems directly measure the tire pressure. Indirect measurement systems measure the displacement of a tire during driving and calculate the tire pressure.
The indirect measurement systems monitor the tire pressure by detecting a decrease in air pressure from a difference in the rotational speed of the left and right wheels with use of a wheel speed sensor used in Anti Lock Brake System (ABS). The systems require substantially no additional cost as long as ABS is installed. However, the systems have drawbacks that air pressure measurement accuracy is lower than direct measurement systems, an air pressure is not detectable if air pressure decrease happens in all four tires, and a measurement error occurs when a tire size is changed, and so on. Therefore, not a few consumers' groups in the U.S. are anxious about monitoring with the indirect measurement systems.
On the other hand, the direct measurement systems measure an air pressure and temperature with a sensor placed in each tire. This system installs a sensor unit in a valve of a tire and monitors all four tires individually. This system therefore has a high monitoring accuracy and allows monitoring of the tire pressure even during parking or stopping. Being more accurate than the indirect systems, the direct systems are expected to prevail over time.
One of the direct measurement systems is a system that measures a tire pressure at regular time intervals, wirelessly transmits the information to a vehicle and displays the information for a driver. This system is composed of a radio transmitter module installed in a tire wheel and a radio receiver module installed in a vehicle body. The transmitter module consists of a sensor module, a signal processor module, and a radio transmitter module. The sensor module includes a plurality of kinds of sensors for detecting pressure, temperature, and so on. The signal processor module processes the data transmitted from the sensor module. The radio transmitter module modulates the data transmitted from the signal processor module and wirelessly transmits the modulated data. Each module is a semiconductor device and thus requires power supply. A battery is generally used to supply power to the modules. The transmitter module is installed inside a tire wheel, and a battery is directly soldered to the transmitter module body to protect from vibration. Replacement of a battery is therefore difficult and thus performed when replacing or discarding the tire. For this reason, improvement in battery life is critical for tire pressure monitoring systems and monitoring methods.
Meanwhile, recent vehicles have more and more electric components and power demand increases accordingly. Lower power consumption is therefore required also for receiver modules installed in vehicle bodies.
A tire internal pressure warning device that aims at solving the above problems is described in Japanese Unexamined Patent Application Publication No. 2003-237327, which is referred to hereinafter as “related art”. FIG. 5 is a block diagram showing a transmitter module of the tire internal pressure warning device of the related art. As shown in FIG. 5, the tire internal pressure warning device 101 has a transmitter module 111 installed in a tire and a receiver module 131 installed in a vehicle.
The transmitter module 111 includes a sensor unit 112, a signal processor unit 113, a transmitter unit 114, and a power source 115. The sensor unit 112 consists of a pressure sensor 121, a centrifugal force sensor 112, a temperature sensor 123, and a voltage sensor 124. The signal processor unit 113 processes the signal detected by the sensor of the sensor unit 112. The transmitter unit 114 transmits a processing result to the receiver module 131. The power source 115 supplies electrical power to the three units.
The signal processor unit 113 generates a clock 1 of 10-second period and a clock 2 of 60-minute period, for example.
The pressure sensor 121 in the transmitter module 111 is driven by an interrupt signal of the clock 1. At this time, a Central Processing Unit (CPU) in the signal processor unit 113 is also driven. Specifically, upon input of the clock, the pressure sensor 121 reads out the previous pressure data stored therein and transmits it to the signal processor unit 113. At the same time, the pressure sensor 121 measures the present pressure data and stores it therein. The signal processor unit 113 determines if the previous pressure data transmitted from the pressure sensor 121 is a predetermined value or higher, thereby determining if the pressure is within the normal range or not. If the pressure data is normal, the pressure sensor 121 is set to standby until the next power supply.
On the other hand, if the pressure value is abnormal, the centrifugal force sensor 122 measures a centrifugal force. If the measurement result shows DRIVE, the signal processor unit 113 reads out the pressure data measured last time and stored in the pressure sensor 121 and transfers it to the transmitter unit 114. The transmitter unit 114 wirelessly transmits the data to the receiver module 131 in the vehicle through the transmitter antenna 125. After that, the pressure sensor 121 enters the standby state and waits for the next synchronous clock to input.
If, on the other hand, the centrifugal force sensor 122 does not detect a centrifugal force, the pressure sensor 121 enters the standby state without performing radio transmission in normal cases. Only in the case where there is an interrupt signal of the clock 2, a start bit period at the start of a radio signal is set longer than a radio signal during driving, radio transmission is performed, and it returns to the standby state. This is because the receiver module 131 is activated intermittently asynchronous with the transmitter unit 114 during stopping and assured data reception is desired.
When the vehicle main switch is on, the receiver module 131 is always in the operation state. When the vehicle main switch is off, the receiver module 131 repeats intermittently the operation state and the standby state not in synchronization with the transmitter unit 114. This technique suppresses power consumption of the transmitter module 111 and the transmitter module 131 by repeating the above operations.
The technique of the related art has the scope for further power consumption reduction. Specifically, though the activation and the standby are repeated in the intermittent activation operation of the power supply 115, this period is not variable. Therefore, the CPU in the signal processor unit 113 and the pressure sensor 121 are intermittently activated in a relatively short period during parking as well as during driving. However, it is sometimes unnecessary to monitor the tire pressure in such a short cycle during parking, and a current consumed in the CPU and the pressure sensor 121 at this time is redundant.